1. Field of the Invention
The present invention relates to an electrostatic ink-jet recording head, particularly to an electrostatic ink-jet recording head for performing recording by ejecting toner onto a recording medium.
2. Description of the Prior Art
Non-impact recording methods are superior to impact recording methods because noises generated during recording are very small. Among the non-impact recording methods, methods using ink-jet recording make it possible to record characters on plain paper with simple mechanisms. In view of these advantages, various ink-jet recording methods have been proposed so far.
A conventional ink-jet recording method is performed by using ink obtained by dispersing toner grains into a carrier liquid, applying a voltage between a pointed recording electrode and an electrode provided behind the recording paper so as to face toward the pointed recording electrode, and ejecting toner grains contained in the ink to the recording paper by the electrostatic force of a generated electric field as disclosed in PCT Publication No. WO 93/11866. FIG. 9 is a perspective view of a conventional ink-jet recording head obtained by modifying the ink-jet recording device disclosed in WO 93/11866, FIG. 10A is an enlarged view of a portion in the vicinity of the ink-jet port (portion a) as viewed from the top of that portion when ink is supplied from the recording head, and FIG. 10B is a cross-sectional view taken along the line b--b in FIG. 10A.
In FIGS. 9 and 10A, 10B, a substrate 101 is an insulator made of plastic or the like, which supports a base film 110 on its upper surface. The base film 110 is an insulator made of polyamide or the like having a thickness of approx. 50 .mu.m and has a plurality of recording electrodes 102 formed integrally with its upper surface. The recording electrodes 102 are obtained by pattern-plating a conductive material such as copper on the base film 110 up to a thickness between 20 and 30 .mu.m and arranged at a pitch of 300 dpi (dots per inch), that is, at an interval of approx. 85 .mu.m. Moreover, each of the recording electrodes 102 independently protrudes beyond an end of the base film 110 between 80 and 500 .mu.m. Furthermore, the surface of each recording electrode 102 is uniformly covered with an insulating coating member 103 having a thickness of 10 .mu.m or less. The base film 110 is formed from a TAB (Tape Automated Bonding) tape and the insulating coating member 103 is formed through chemical vapor deposition of Parylene.
A cover 104 is set on the base film 110 so that it does not cover the protrusions of the recording electrodes 102. The cover 104 is an insulating member on which an ink supply port 105 and an ink discharge port 106 are previously formed. A space formed with the base film 110 and the cover 104 constitutes an ink chamber which is filled with ink supplied from the ink supply port 105. Moreover, the front end of the cover 104 opens and a slit-like aperture defined between the base film 110 and the cover 104 forms an ink jet port 107 on which an ink meniscus 108 are formed.
Ink forms the ink meniscus 108 on the ink jet port 107 according to its surface tension as shown in FIGS. 10A and 10B. Because a negative pressure is applied to the ink in the head and the recording electrodes 102 protrude beyond the base film 110 and cover 104, the ink meniscus 108 has a diagonally downward concave shape when viewed from its side. Moreover, because the recording electrodes 102 individually protrude to the outside of the ink jet port 107, an ink meniscus 108 is formed correspondingly at each recording electrode 102.
Therefore, when applying a high-voltage pulse to a selected recording electrode 102, an electric field is concentrated on the protruded front end of the recording electrode 102 having an ink meniscus formed thereon. The electrified toner in the ink is propelled by the electric field and discharged from the protruded front end toward an electrode (not illustrated) facing the ink-jet recording head, that is, in the direction of the recording paper, as toner agglomerations 109.
Other ink-jet recording devices using electrostatic force are disclosed in the official gazette of Japanese Patent Application Laid-Open No. 58-124662, issued on Jul. 25, 1983, and the official gazette of Japanese Patent Application Laid-Open No. 56-167473, issued on Dec. 23, 1981.
In the case of the ink-jet recording device disclosed in the official gazette of Japanese Patent Application Laid-Open No. 58-124662, the discharge point of an ink discharge port is formed on the front end of a separation wall for defining an ink channel. The separation wall is formed along a recording electrode, and the discharging point is formed at the end of the recording electrode.
In the case of the ink-jet recording device disclosed in the official gazette of Japanese Patent Application Laid-Open No. 56-167473, a division plate for dividing an ink channel is formed in the ink channel. The division plate has a plurality of recording electrodes on both surfaces thereof. The front end of the division plate is formed so as to protrude beyond an ink discharge port.
A first problem of these conventional electrostatic ink-jet recording heads lies in the fact that the quantity of toner grains for forming a desired dot are not sufficiently supplied to the discharge point. This is because the discharge point is formed by a recording electrode, wherein the electrostatic force directed toward the discharge point does not affect a sufficient amount of toner grains near the discharge point when a recording voltage is applied. Therefore, not enough toner grains to form a desired dot is concentrated on the discharge point.
A second problem in the prior art lies in the fact that ink droplets are unstably discharged. This is because the plurality of convex ink meniscuses using the discharge points as apexes are continuously connected to each other, whereby vibrations of the liquid surface near a discharge point generated while discharging ink influence the ink meniscuses of other discharge points. Therefore, the ink meniscuses cannot always be stably obtained.
A third problem lies in the fact that ink droplets are irregularly discharged due to concentration of excessive toner grains. This problem occurs in the prior art shown in FIGS. 9 and 10A, 10B and the official gazette of Japanese Patent Application Laid-Open No. 56-167473. This is because the discharge aperture for supplying ink to the discharge point is formed as a slit on a part of the ink chamber so as to prevent ink from overflowing. Therefore, the ink does not flow in the discharge aperture, causing the ink viscosity to increase due to concentration of excessive toner grains in the discharge aperture.
The present invention is made to solve the above problems by providing an electrostatic ink-jet recording head for discharging ink droplets containing toner grains in an electrostatic field, wherein the recording head is capable of preventing the problem of an insufficient amount of toner grains for forming a desired dot, stably discharging ink droplets, and moreover preventing ink droplets form being irregularly discharged due to a concentration of excessive toner grains in a discharge aperture.